Download Chapter 6 Are You Only as Smart as Your Genes?

Chapter 6
Are You Only as Smart as Your Genes?
Mendelian and Quantitative Genetics
Copyright © 2007 Pearson Prentice Hall,
1 Inc.
Copyright © 2007 Pearson Prentice Hall, Inc.
6.1 The Inheritance of Traits
• Most children are similar to their parents
• Children tend to be similar to siblings
• Each child is a combination of parental traits
• The combination of paternal traits and maternal
traits is unique for each individual child
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The Inheritance of Traits
The human life cycle:
• gametes (a male sperm cell + a female egg cell)
fuse during fertilization to form a single celled
zygote, or embryo
• the embryo grows by cell division in mitosis
• the embryo grows into a child
• the child matures into an adult
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The Inheritance of Traits
The human life cycle:
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Genes
• Genes are segments of DNA on chromosomes
– that carry information about how to make proteins
– Structural proteins
• for things like hair and cell “skeletons”
– Functional proteins
• for things like digesting food and making the cell cycle
go or stop
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Genes
• All human cells have the all the same genes
– Same 46 chromosomes in EVERY cell of an individual
• Only certain genes are active in a single cell type
– Heart cells and eye cells have genes for the protein
rhodopsin, which helps to detect light
– Rhodopsin protein is only produced in eye cells,
not heart cells
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Genes and Chromosomes
• DNA is sort of like an instruction manual that shows
how to build and maintain a living organism…
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Genes Are on Chromosomes
• The genes are located on the chromosomes
• The number of chromosomes depends on the
organism
– Bacteria
• one circular chromosome
– Humans
• 23 homologous pairs of linear chromosomes
• 46 total chromosomes
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Genes Are on Chromosomes
• Each of the 23 pairs of chromosomes
– is a homologous (same) pair that carry the same gene
• For each homologous pair
– one came from mom and the other from dad
• Gene for earlobe shape from mom and gene for earlobe shape from dad
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Gene Variation Is Caused by Mutation
• Genes on a homologous pair are the same, but the
exact information may not be the same
– Earlobe shape from mom is “attached”
– Earlobe shape from dad is “detached”
• Same gene for earlobe shape, different versions or
alleles of that same gene
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Gene Variation Is Caused by Mutation
• Originally in evolution, all genes were the same
– no different alleles
• Sometimes there are errors or mutations in DNA
• Mutations in genes can cause somewhat different
proteins to be produced
– Causing detached earlobes instead of attached earlobes
• These different gene versions create alleles
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Diversity in Offspring
• The combination of alleles from the parents
– create the individual traits of each child
• Differing alleles from parents are the primary
reason that non-identical twin siblings are not the
same
– Environment can also play a role
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Diversity in Offspring
Non-identical twin siblings:
• The combination each individual receives
depended on the gametes that were part of the
fertilization event
– Which egg and which sperm
• Remember:
– that each gamete has only 1 copy of each homologous
chromosome pair with one allele
• Either the allele from that person’s mom or dad
• but not both
– (normal cells have 2 homo chromos = 2 alleles)
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Law of Segregation
• Separation of the 2 alleles of each gene during gamete
formation is called the law of segregation
• When a gamete is formed
– the 23 homologous pairs are separated (46 total chromos)
– so that one of each pair goes into one gamete (23 chromos)
• One chromo #1, one chromo #2, one chromo 3#, etc
– and the other into another gamete (23 chromos)
• This results in gametes with only 23 chromosomes
– 1 of each of the 23 homologous pairs
– half of the 46 chromosomes
– half of the alleles
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Independent Assortment
• Due to independent assortment
– parents contribute a unique subset of alleles to each of their nonidentical twin offspring
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Independent Assortment
Another independent assortment analogy:
• A pair of shoes is comparable to a
homologous pair of chromosomes
– The offsrping shown to the right was made
from one gamete with all R shoes but one L
and a Gamete with all L shoes but one R
• Meiosis separates the members of one
pair independently of other pairs
–
–
–
–
Some gametes may have all R or all L shoes
Others may have half R ad Half L
Others may have 3/4 R and 1/4L
Etc
• Since each gamete is produced
independently of the others
– the combination of chromosomes (shoes) and
corresponding genes is unique
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Diversity in Offspring
• That means a unique egg will be fertilized by a unique
sperm to produce a unique child
• For each gene, there is a 50% chance of having the same
allele as a sibling
• 50% chance of getting a R shoe or a L shoe
• If siblings both got the same shoes, they will be similar.
• If they both got the opposite shoes, they will be totally
different
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Diversity in Offspring
• There are 223 combinations for the way the
homologous chromosomes could line up and
separate
– This is more than 8 million combinations
• Gametes combine randomly
– without regard to the alleles they carry in a process
known as random fertilization
• You are one out of 64 trillion genetically different
children that your parents could produce
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Diversity in Offspring
• The main ways sexually reproducing organisms
result in offspring with unique combinations of
alleles
– Segregation of Alleles
– Independent assortment of genes
– Random fertilization
• These processes produce the genetic diversity of
individuals found in humans
– and all other sexually reproducing biological populations
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Twins
• Fraternal (non-identical)
– Dizygotic
• two separate fertilized eggs at the same time
–
–
–
–
Not genetically the same
Same as non-twin siblings
Just happen to be in the uterus at the same time
Results from 2 ovulated eggs
• Usually only one egg is ovulated per cycle
• More than one egg ovulated per cycle can result from:
– Genetic predisposition
– Fertility drugs
– First cycle after cessation of prolonged birth control pill use
– Super-ovulation in women over 30
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Twins
• Identical
– monozygotic:
• one single fertilized egg that separates into two embryos
– genetically the same
– Identical alleles
– A natural “clone”
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